It may be necessary to encapsulate a piezoelectric element in order to protect it from potentially damaging environmental conditions. For example, in one known application, described in EP 0 995 901, a piezoelectric element is surrounded with high pressure fuel. However, the fuel and the piezoelectric element must be kept separate in order to prevent any moisture present within the fuel having an electrochemical effect on the compounds in the piezoelectric material.
In many applications, such as that described in EP 0 995 901, the material encapsulating the piezoelectric element must be flexible in order to enable the piezoelectric element to change in length. At present preferred materials for the casing are polymers as these exhibit many desirable characteristics. However, polymers are not an ideal material because they cannot provide a permanent moisture barrier, due to their inherent permeability. Even in the most permeation resistant polymers, such as fluorinated polymers or elastomers, water permeation is significant.
Moisture that has entered the casing of a piezoelectric element at the manufacturing stage can also cause problems. This moisture has the same potential to damage the piezoelectric element as that permeating through the casing and therefore it is desirable to remove it.